This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 100 12 395.3 filed in Germany on Mar. 15, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a method for measuring a flow rate of a flowing medium, and to a flow meter.
It is known to use ultrasound to determine flow rates of a flowing medium, in particular of gas. The medium is irradiated with a sound wave via a microphone, the sound wave transmitted by the medium being detected by means of a second microphone, which is arranged downstream or upstream. Piezoelectric or capacitive vibration emitters or pick-ups are preferably used as microphones. There are two measurement methods in principle. In a first method, the time interval between emission and detection of the sound wave is determined, and the flow velocity is calculated therefrom. In a second method, the Doppler effect which is caused by the flowing medium is utilized and its frequency shift is determined. Such an ultrasonic measurement is disclosed, for example in U.S. Pat. No. 4,080,837.
Measurement by means of ultrasound has the disadvantage that the microphones emitting sound must be in direct contact with the medium. Consequently, tube walls must be provided with appropriate openings in which the microphones are arranged. Since the media to be measured are also often aggressive, the microphone must be appropriately provided with a resistant protective layer. Furthermore, the microphone or its protective layer is exposed to deposits which impair the sound emission.
In the prior art, furthermore, a photoacoustic method is known for detecting pollutants in a medium. Use is made in this photoacoustic method of the fact that the pollutants and the medium absorb optically at different wavelengths. The polluted medium is therefore irradiated with focused light of a defined wavelength which is absorbed by the pollutant, but not by the medium. If pollutants are present in an irradiated volume, the absorption leads to local heating, whereupon the volume expands and emits a pressure and sound wave. This sound wave can be detected by means of microphones. Such photoacoustic methods are disclosed, for example, in DE-A-31,39,917 and EP-A-0,908,717.
It is the object of the invention to create a method for measuring a flow rate, and a flow meter of the type mentioned at the beginning, which eliminate the disadvantages of ultrasonic measurement.
In the method according to the invention, a photoacoustic effect is utilized to produce a sound wave in the medium. In this process, the medium is irradiated with light, a wavelength being used which is absorbed by the medium. The medium therefore produces the sound wave itself.
The light can be guided through a small tubular opening from a light source into the medium, so that only a minimum area need be kept clean. In addition, the flow meter can be fashioned smaller than the known flow meters based on ultrasound.
In a first variant of the method, the propagation time of a photoacoustic sound wave is measured in order to determine a flow velocity. In a preferred subvariant, a propagation time of the sound wave in the flow direction is compared with a propagation time counter to the flow direction in order to obtain a measurement independent of an instant of the generation of the sound wave.
In a second variant, a Doppler frequency change in the photoacoustic sound wave is detected in order to determine a flow velocity. In a preferred subvariant, the Doppler frequency change in the flow direction is compared with a Doppler frequency change counter to the flow direction, in order to obtain a measurement independent of an instant of the generation of the sound wave.
In a third variant of the method, the propagation time of the photoacoustic sound wave is measured independently of the flow velocity, in order to determine the density of the medium.
In a fourth variant of the method, a flow profile is set up by irradiating the medium with light beams which have focal points separated spatially from one another, an array being formed by means of the focal points.